As portable, small electric and electronic devices become more widespread and improve in performance, the production of non-aqueous electrolyte secondary batteries typified by lithium ion secondary batteries has greatly increased, and improvements in their capacity and cycle life are continuously demanded.
At present, in typical non-aqueous electrolyte secondary batteries, carbon materials are primarily used as negative electrode materials. However, in negative electrodes made from carbon materials, Li can only be occluded up to the composition LiC6. Therefore, the theoretical maximum limit on the capacity is 372 mAh/g, which is only approximately {fraction (1/10)} that for the case for metallic lithium, and there is a limit on increases in capacity.
Metallic lithium, which was initially used as a negative electrode material, can provide a high capacity, but repeated charging and discharging of a battery cause the precipitation and growth of dendrite crystals, leading to the occurrence of short-circuiting, so the cycle life of charging and discharging was short, and it was not practical.
With the object of obtaining a high capacity, it has been proposed to use the element Al, which can reversibly occlude and release Li by the formation of an intermetallic compound, as a negative electrode material. However, due to changes in volume accompanying occlusion and release, the negative electrode material tends to form cracks which causes powderization or comminution of the material into a fine powder. Therefore, in secondary batteries using this negative electrode material, as cycles of charging and discharging progress, the capacity abruptly decreases, so they have a short cycle life.
As a measure to prevent this powderization of a negative electrode material caused by changes in volume, it has been proposed to add Li, Si, B, or the like to Al in a negative electrode material in order to increase the lattice constant of the aluminum material in advance (Japanese Published Unexamined Patent Application No. Hei 3-280363). However, the effect is inadequate, and it is not possible to sufficiently increase the cycle life.
It has also been proposed to occlude and release Li within the lattice of silicides or other intermetallic compounds (Japanese Published Unexamined Patent Applications Nos. Hei 7-240201, Hei 9-63651, etc.), but in each case, a significant effect was not obtained.
Various types of negative electrode materials for non-aqueous electrolyte secondary batteries and negative electrodes formed from those materials have been proposed, but a negative electrode material having a structure which can best exhibit the performance of those materials and a process for its manufacture have not been proposed.